Detecting an open neutral condition in the power supply to an electrical circuit can improve the safety of the powered circuit and connected devices. In some cases, open neutral protection may be required by any of a variety of regulating organizations, such as, for example, Underwriters Laboratories (UL) or Canadian Standards Association (CSA).
A common electrical safety option is the use of ground fault circuit interrupters (GFCIs). A GFCI is a fast-acting circuit breaker that can sense small imbalances in the circuit caused by leakage to ground and opens the circuit to disconnect the power supply from the circuit and any connected devices. A GFCI continually monitors the amount of current returning from the protected circuit and any connected devices along the normal electrical path. Whenever the amount of current “returning” differs from the amount “going” by a specified amount, the GFCI interrupts the flow of current—preventing damage and potential electrocution.
GFCIs rely on a properly connected and conducting neutral line to work correctly. For example, if the power side neutral conductor is open or lifted at the supply panel (an “open neutral” condition), the circuitry in the GFCI that senses a ground fault will not have the necessary power from the power supply to operate. In this situation, the GFCI is no longer capable of sensing and disengaging the power in the event of a ground fault. However, the hot line of the power supply may be properly connected and conducting. A user of the circuit will not be protected by the disabled GFCI device and if a faulty device is connected to the unprotected circuit, the user may be exposed to a shock or electrocution hazard.
An open neutral protection device can help ensure proper operation of downstream GFCIs and protect users of these circuits by disconnecting both hot and neutral power supply lines whenever either of these supply lines is disconnected or broken. Certain power distribution equipment may be required to include open neutral protection by, for example, UL standard 943 or UL standard 1640. In addition, for example, the National Electric Code requires open neutral and GFCI protection for lighting and other equipment used in and around swimming pools and spas, and for temporary power being supplied to worksite locations.